Emx1-Cre is expressed in vagal afferent nerves that regulate central cardiorespiratory functions

Abstract

The Emx1-IRES-Cre transgenic mouse is commonly used to direct genetic recombination in forebrain excitatory neurons. However, the original study reported that Emx1-Cre is also expressed embryonically in peripheral autonomic ganglia, which could potentially affect the interpretation of targeted circuitry contributing to systemic phenotypes. Here, we report that Emx1-Cre is expressed in the afferent vagus nerve system involved in autonomic cardiorespiratory regulatory pathways. Our imaging studies revealed expression of Emx1-Cre driven tdtomato fluorescence in the afferent vagus nerve innervating the dorsal medulla of brainstem, cell bodies in the nodose ganglion, and their potential target structures at the carotid bifurcation such as the carotid sinus and the superior cervical ganglion. Photostimulation of the afferent terminals in the nucleus tractus solitarius (NTS) in vitro using Emx1-Cre driven ChR2 reliably evoked excitatory postsynaptic currents in the postsynaptic neurons with electrophysiological characteristics consistent with the vagus afferent nerves. In addition, optogenetic stimulation targeting the Emx1-Cre expressing structures identified in this study, such as vagus nerve, carotid bifurcation, and the dorsal medulla surface transiently depressed cardiorespiratory rate in urethane anesthetized mice in vivo. Together, our study demonstrates that Emx1-IRES-Cre is expressed in the key peripheral autonomic nerve system and can modulate the cardiorespiratory function independently of forebrain expression. These results raise caution when interpreting the systemic phenotypes of Emx1-IRES-Cre conditional recombinant mice, and also suggest the utility of this line to investigate the modulators of afferent vagal system. Significance StatementEmx1-IRES-Cre mice are widely used to dissect critical circuitry underlying neurological disorders such as epilepsy. These studies often assume the Cre is expressed selectively in forebrain excitatory neurons. However, earlier work reported that Emx1 is expressed in several peripheral tissues of the developing embryo and thus gene recombination may affect these peripheral structures. In this study, we characterized the expression and physiological functions of Emx1-Cre expressed in the vagus nerve, the critical peripheral autonomic component. Optogenetic stimulation of these Emx1-Cre+ neurons activates the nucleus tractus solitarius neurons within the brainstem in vitro and induces complex cardiorespiratory reflex in vivo. This study confirmed that peripheral Emx1-Cre+ cells are involved in autonomic regulation and potentially affect transgenic mouse phenotypes.